The present invention pertains to the recirculation of cabin air in an air conditioning system for a pressurized aircraft of the type in which fresh air is initially supplied to the cabin by bleeding air from one or more of the aircraft's engines and then conditioning such bleed air before introducing it into the pressurized fuselage shell.
Most turbojet aircraft used by commercial carriers and by the military, have cabin air conditioning systems that tap engine bleed air, condition it in a refrigeration pack, and then introduce the fresh air into the pressurized compartments of the aircraft. Typically, the fresh air conditioning is done in an Air Cycle Machine (ACM) refrigeration pack in which the bleed air is cooled by a combination of exchanging heat between the hot bleed air and cooler outside ambient air, and extracting work from the bleed air. First, the bleed air is compressed, and it is then allowed to expand in an expansion turbine, which cools the bleed air and in doing so performs work that is harnessed to drive the compressor. In some modern versions of ACM (three-wheel machines) part of the turbine work is absorbed by a fan that induces ambient air through the heat exchangers. In other aircraft air conditioning systems the bleed air is conditioned by a Vapor Cycle Refrigeration pack.
It has long been recognized that the use of bleed air in conjunction with ACM refrigeration has a number of shortcomings, including a primary disadvantage resulting from the amount of fuel that must be burned by the engines to support the aircraft's air conditioning system. The ACM expends fuel both in terms of the amount of engine bleed air that must be supplied to it, and in terms of the amount of drag on the aircraft attributable to the quantity of cooler ambient air that must be ingested for providing heat sinking for the bleed air. Although numerous improvements have been proposed, such as disclosed in U.S. Pat. Nos. 3,177,679 by D.F.J. Quick et al., in an effort to improve the overall efficiency of this type of aircraft air conditioning system, the inherent characteristics of cooling engine bleed air by an ACM result in a significant fuel burn penalty that thus far has not been satisfactorily overcome.
The above disadvantage of available air conditioning systems becomes even more acute when they are used in conjunction with today's high-bypass ratio engines, as contrasted with the now less popular low-bypass ratio engines. A high-bypass ratio engine has significantly greater constraints on the amount of bleed air that can be safely tapped from the engine before seriously interferring with the engine's performance. Thus, in order to provide a suitable cabin ventilation rate it is necessary to compete with the limited availability of bleed air that can be supplied by a high-bypass engine while still maintaining optimum performance.
Improvements have been proposed for reducing the dependence of cabin ventilation on the availability of engine bleed air to reduce operating penalty and engine thrust loss. For example, on the Model 747 aircraft, manufactured by The Boeing Company of Seattle, Wash., passenger cabin air is recirculated by electrically-powered ventilation fans that withdraw stale air from the cabin, force it through air filters and return it to the cabin. The electrically-powered recirculation fans do not provide additional cooling capacity. They in fact, impose additional cooling load on the ACM due to the energy they expend in recirculating cabin air. Also, the electrical power consumed by the recirculation fans adds to the fuel-burn penalty attributed to the air conditioning system since the on-board electrical generators are powered by the aircraft engines during flight.
Other known air conditioning systems have provided for recirculation of cabin air by withdrawing stale air from the cabin, compressing it and mixing the thusly compressed recirculation air in with the primary flow of fresh air derived from engine bleed air. Typically, the mixing point for the bleed air and the compressed cabin air is at a point upstream of one or more expansion turbines associated with an ACM refrigeration pack. Various versions of this kind of cabin-air recirculation system are disclosed in U.S. Pat. Nos. 2,477,931 issued to King; 2,491,462 issued to Wood; 4,021,215 issued to Rosenbush and 3,097,508 issued to Leech et al. Like the fan-powered recirculation system discussed above, the use of a compressor to recirculate the cabin air as part of the primary ACM refrigeration system achieves air recirculation only at the expense of fuel burn.
In other air conditioning systems, it is known to use the potential energy available from the differential pressure that exists between a pressurized aircraft cabin and ambient air, in order to power, at least in part, certain of the fresh-air supplying components of an air conditioning system. However, a turbine powered in this manner is typically used to augment the drive for one or more of the compressor stages of the primary fresh-air supplying system, such as an ACM, or to drive a simple fresh air supply that draws in fresh, ambient air. Examples of air conditioning systems in which the primary supply is in part powered by a cabin-to-ambient pressure differential are disclosed in U.S. Pat. Nos. 2,767,561 issued to Seeger; 2,734,356 issued to Kleinhans; and 2,777,301 issued to Kuhn.
Accordingly, it is an object of the present invention to reduce, in an overall efficient manner, the amount of engine bleed air required to supply conventional, aircraft cabin air conditioning systems used on pressurized aircraft without degrading ventilation rate. A related object is to provide a net reduction of the fuel burn needed to operate the aircraft's air conditioning system at an operational level that supplies adequate amounts of conditioned air to the passenger cabin for pressurization, heating and cooling, and circulates such air for adequate smoke removal and ventilation.
Another object is to provide a cabin air conditioning system that achieves the foregoing objectives and is capable of being easily and inexpensively retrofitted to pressurized cabin aircraft having ACM or Vapor Cycle Refrigeration packs.
Additional objects and advantages of the invention are to provide method and apparatus for conditioning recirculation air in a manner that does not require any additional, external power, electric or pneumatic, for its operation; and does not require a complex or expensive control system.